This invention relates generally to hand held soldering instruments and more specifically to electronically controlled soldering irons whose tip temperature is sensed and controlled by electronic control means which is thermally isolated from the tip.
In the electronics field, both in production facilities as well as in laboratories for research, development or repair, it has become essential to have soldering and desoldering tools capable of providing precise soldering tip temperatures in order to minimize the risk of damage to thermally sensitive components affixed by the solder to other circuit elements. From non-controlled soldering irons, the art has developed through crude bimetalic strip controls to thermistor or thermocouple controlled instruments and more recently to tools electronically controlled by a circuit responsive remotely to the thermally dependent resistance of the heating element itself. This technique is described in U.S. Pat. No. 4,507,546, issued Mar. 26, 1985, to William S. Fortune and Wayne A. Murray. In essence, the power leads carrying the electric current through the soldering tip heater are used, for a very short period of each cycle during the inflection point of the applied alternating current, to effect a resistance measurement. This may be done to provide a signal which is a function of the temperature of the heater as often as every cycle while still impressing full wave power to the heater.
The best known prior art examples of electronically controlled soldering instruments employ sophisticated control circuitry housed within the handle of the soldering tool. Such instruments provide significant advantages over the previous art; however, they suffer limitations caused by the proximity of the electronic control components to the heating element, the difficulties of dissipating the heat generated by the control circuit itself, and the necessity of carrying the volume and mass of the control circuitry in the hand of the operator.
The difficulties of providing control circuit stability, accuracy, and component longevity in the environment of a heated soldering iron are obvious. It is also clear that such thermal problems are further aggravated by the heat generated by the control circuit itself and the difficulties of dissipating it away from the severly confined interior of the tool handle which, obviously, in addition to being reasonably cool must be kept as small and light as possible in order to be practical for long periods of operator use.
A further number of disadvantages include the relatively high cost of providing an instrument which successfully optimizes these problems with the advantages of a reliable and precise soldering tool. It may also be noted that a realistic item of cost is the vulnerability of the precision tool to damage from dropping or bumping it particularly since the electronics inherently are exposed to the risks of a hand tool fastened to the end of power cord likely to be inadvertently snagged or pulled.
Another limitation of prior art soldering tools is their size: irrespective of expense, the smallest available electronically controlled soldering instrument is too large and unwieldy for many otherwise desireably useful applications such as, for example, soldering or desoldering under a microscope or in other very confined workspaces.
Accordingly, it is an object of the present invention to provide an electronically precision controlled soldering instrument which does not suffer these and other disadvantages and limitations of the prior art.
It is another object to provide such a soldering instrument which is significantly lighter and smaller and less expensive than was heretofore available.
It is another object to provide such an instrument which is invulnerable to control circuitry damage due to soldering tip heater thermal effects or physical abuse to the hand held tool.